A DC to AC inverter has a DC source and an output transformer with a center tapped primary winding. Two switches connect the DC source alternately with each half of the primary winding generating an alternating signal in the transformer secondary. A pulse width modulator is responsive to the inverter output to control the relative time of conduction of each of the switches. The modulator includes a clock which generates a ramp signal that is compared with an output error siganl, generating switching pulses that are directed alternately to the two switches.
In some situations it is desirable that the inverter clock be synchronized with an external signal source so that beat frequencies are not set up in the circuits of other electronic equipment located in proximity to the inverter.
If synchronization occurs near the start of a ramp signal, there will be two nearly successive actuations of the same switch within a period of the clock. This results in an unbalance in the flux conditions of the output transformer and the transformer may saturate causing a current overload and failure of one of the switches.